The present invention relates to temperature sensitive surfaces, methods of bonding functional surface materials such as poly(N-isopropylacrylamide)(PNPIPAAm) to substrates and applications in microtechnology and anti-fouling.
The ability to control surface properties has, for many years, been an important goal for materials scientists working in a wide variety of areas. Many techniques have been explored to modify surfaces, including plasma treatment, corona discharge, ozone treatment, electron-beam bombardment, and ultraviolet (UV) and X-ray irradiation. Surface properties can be modified by applying a polymer, such as by photopolymerization. See, for example, C. G. Roffey, Photopolymerization of Surface Coating, John Wiley and Sons, New York, 1982, Chapter 3. Poly(N-isopropylacrylamide) (PNIPAAm) is an especially interesting and important polymer for controlling surface polymers. This polymer, and related polymers such as copolymers comprising the isopropylacrylamide (IPAAm) monomer, exhibit thermally reversible changes in response to temperature changes. More particularly, in an aqueous system (i.e., in water), increasing the temperature of surfaces modified by PNIPAAm changes the surface from hydrophilic (water-loving) to hydrophobic (water-repellant). The temperature at which the surface changes from hydrophilic to hydrophobic is known as the lower critical solution temperature (LCST). Attachment to various surfaces and properties of surfaces modified with PNIPAAm has been described in numerous references including: T. Yakushiji, et al., Langmuir, 1998, 14, 4657; T. Okano et al., J. Controlled Release 1995, 36, 125; and L. Liang et al, J. Appl. Polym. Sci., 1999, 72, 1. Grafting PNIPAAm within a capillary tube or sponge has been identified as a potentially intelligent switch and control valve, respectively. See L. Liang, et al., Macromolecules, 1998, 31, 7845; L. Liang et al., J. Appl. Polym. Sci., 2000, 75, 1735.
Despite previous efforts toward developing thermally responsive and reversible surfaces, there remains a need for thermally responsive surfaces that have relatively thin, polymeric surface coatings and yet exhibit desirable properties such as: narrower ranges for the hydrophilic/hydrophobic change, relatively low contact angles at low temperature, high contact angles at high temperature, a large change in contact angles across the LCST, high grafting density and fast property changes in response to temperature changes. There is also a need for new methods of making thermally responsive surfaces that are simpler, less expensive, use fewer materials, and/or result in surfaces having superior properties.
The present invention provides an article having a thermally sensitive surface in which a thin coating is disposed over a substrate. The thin coating has a thickness of less than 50 nm and comprises the reaction product of IPAAm. This thermally sensitive surface is characterized by a LCST of at least 25xc2x0 C. and an advancing contact angle of less than 60xc2x0 at low temperature and greater than 80xc2x0 at high temperature. The use of the term xe2x80x9clow temperaturexe2x80x9d in this application refers to a temperature above 0xc2x0 C. but below the LCST, while xe2x80x9chigh temperaturexe2x80x9d refers to a temperature below 100xc2x0 C. but above the LCST. For the present invention, the advancing contact angle is measured according to the technique described herein in the Examples section. The invention can comprise a variety of shapes and conformations; however, the contact angle should be measured on an essentially flat area of the temperature sensitive coating. If the article lacks any essentially flat areas, the contact angle should be measured on a test surface that is flat but otherwise identical to the article in question.
The present invention further provides an article having a thermally sensitive surface in which a cross-linked coating is disposed over a substrate. This coating is the reaction product of a composition that includes IPAAm and a crosslinking agent. The temperature sensitive surface is characterized by an advancing contact angle of less than 20xc2x0 at low temperature and greater than 80xc2x0 at high temperature.
The invention also provides methods of forming thermally sensitive surfaces. In one method, a photosensitizer is reacted onto a surface to provide a reactive surface; a composition comprising IPAAm is applied onto the reactive surface; and the composition is photopolymerized to result in a thermally sensitive surface having a thin polymeric coating having a thickness of less than about 50 mn. This thin coating has an advancing contact angle of less than 60xc2x0 at low temperature and greater than 80xc2x0 at high temperature.
In another method, a surface modifier is reacted onto a surface to provide a reactive surface; a composition comprising IPAAm and a crosslinking agent is applied onto the reactive surface; and the composition is thermally polymerized to result in a temperature sensitive surface having a crosslinked polymeric coating having an advancing contact angle of less than 20xc2x0 at low temperature and greater than 80xc2x0 at high temperature.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.